(1) Field of the Invention
The present invention relates to a battery control device, in particular, to a battery control device including: an equalizing means for equalizing a voltage of a plurality of unit cells connected together in series forming an on-vehicle high-voltage battery; and a low-voltage control means which receives electric power from an on-vehicle low-voltage battery having a supply voltage lower than that of the on-vehicle high-voltage battery and outputs an equalization command to the equalizing means.
(2) Description of the Related Art
Recently, a hybrid electric-internal combustion vehicle (hereinafter, HEV) which travels by using both an engine and an electric motor has been widely developed. The hybrid motor vehicle includes: an on-vehicle low-voltage battery (hereinafter, low-voltage battery) having a voltage of about 12 V for starting the engine; and an on-vehicle high-voltage battery (hereinafter, high-voltage battery) for driving the electric motor. The high-voltage battery provides a high voltage by connecting a plurality of unit cells in series, the unit cell being a secondary battery such as a nickel-hydrogen battery or a lithium battery.
As for the high-voltage battery, dispersion (i.e. non-uniformity) of a voltage of each unit cell, that is, dispersion of a state of charge (SOC) occurs by repeating charge and discharge. Therefore, upon charge and discharge of the battery, from the viewpoint of durability and safety of each cell, it is necessary to prohibit a charge at a time point when a voltage of a unit cell having the highest SOC (i.e. voltage) reaches a predetermined upper limit SOC (i.e. upper limit voltage value) and at a time point when a voltage of a unit cell having the lowest SOC (i.e. voltage) reaches a predetermined lower limit SOC (i.e. lower limit voltage value). Therefore, when dispersion of SOC occurs for each unit cell, a usable capacity of the battery is substantially decreased. As for an HEV, this undesirable phenomenon renders assist and regeneration insufficient, wherein the assist means a supplement of battery energy with respect to gasoline upon hill-climbing and the regeneration means regeneration of energy to the battery upon hill-descent, thereby causing deterioration in a vehicle dynamic characteristic or fuel efficiency.
As for the problem described above, proposed is a charge-type equalizing device for removing dispersion of a voltage of each cell, in which a minimum unit cell having the lowest voltage is extracted from among a plurality of the unit cells, the unit cells except the minimum unit cell are connected to a discharge resistance, and stored charges are subjected to a discharge until a voltage of the connected unit cells reaches the voltage of the minimum unit cell (for example, Japanese Patent Application Laid-Open No. 2002-199510).
Also, proposed is a charge pump-type equalizing device for removing dispersion of a voltage of each cell, in which charges are shifted from a unit cell having high voltage to a unit cell having low voltage through a capacitor (for example, Japanese Patent Application Laid-Open No. H10-225005 and Japanese Patent Application Laid-Open No. 2004-120871).
The equalizing device described above performs an equalizing action by receiving electric power from a low-voltage battery while an ignition switch is turned off, that is, while an engine is turned off. The low-voltage battery is never charged while the engine is turned off. Further, while the engine is turned off, the low-voltage battery supplies electric power to circuits (for example, a circuit for detecting a charge or discharge current of the battery or a circuit for detecting temperature of the battery) other than the equalizing device. Therefore, the low-voltage battery is subjected to an excess discharge while the ignition is turned off, thereby causing the battery to discharge completely (i.e. to go dead) and therefore, causing the engine to fail to start.
Therefore, proposed is an equalizing device, in which when a voltage of a low-voltage battery is deteriorated, the equalizing action is stopped (for example, Japanese Patent Application Laid-Open No. 2003-189490). However, in this case, there is a problem that the equalization, which originally should be carried out while the ignition is turned off, is not carried out, such that the dispersion of the voltage of each unit cell is not removed.